Async refresh

Move refresh of google music content to an extra thread.
This fixes two problems:

1. Very long startup times on small systems, like the raspberry pi
2. Remote changes to the content, like new/changed playlists
Is there a reason you want to do that? If you start both thread in that order, the playlist refresh thread will start after the library refresh one, so you'll end up with the same behavior, without the extra skip logic you added in _refresh_playlists.